The reason SSC is not used on USB is because the 48MHz is used for frame
synchronization. It is specifically required not to modulate the 48MHz clock
and the reference clock in Intel's clock generator specs.

I am not sure about the specific Cypress part you are referring to. It may
very well have fixed USB output when other output clocks are modulated, just
like most other SSC generators do.

Michael,
What do you mean SSC can NOT be used on any
fixed frequency clocks like USB, actually, I read
an SSC's datasheet from cypress. It does have a
dedicated output for USB.

Regards,
Yu Wang

--- "Zhang, Michael T" <michael.t.zhang@intel.com>
wrote:
> Ravinder,
>
> As you said, SSC is a percentage frequency
> modulation. So it spreads the
> higher harmonic of clock frequency over a wider MHz
> range, therefore, brings
> a larger amount of EMI reduction of the peak
> amplitude, NOT higher emission.
> This has been seen in actual PC systems. In
> addition, SSC benefit increases
> at frequencies over 1GHz due to the use of average
> detector.
>
> SSC itself does not introduce more jitter because of
> the small percentage of
> frequency modulation. This is the reason it has been
> successfully used with
> RDRAM, which has 400MHz differential clocks with
> very tight jitter
> requirement. However, SSC can not be used on any
> fixed frequency clocks,
> like USB.
>
> -Michael T. Zhang
> Processor Product Development
> Performance Microprocessor Division, Intel Corp.
> (503) 264-2301
>

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